The present invention relates to a process for producing xcex5-caprolactam and, in particular, a process for producing a high-purity xcex5-caprolactam by crystallizing xcex5-caprolactam from crude xcex5-caprolactam which contains impurities.
xcex5-Caprolactam is an important compound is which used as an intermediate for the preparation of polyamides such as Nylon-6, and many processes are known to produce xcex5-caprolactam. For example, xcex5-caprolactam has been produced by subjecting cyclohexanone oxime to Beckmann rearrangement in the presence of an acidic medium such as fuming sulfuric acid. This process has drawbacks including the production of a large amount of ammonium sulfate.
The above process is improved by gas phase Beckmann rearrangement using a solid catalyst. Solid catalysts used for the gas phase Beckmann rearrangement include boric acid catalysts, silica-alumina catalysts, solid phosphoric acid catalysts, complex metal oxide catalysts, zeolite catalysts, etc. Furthermore, JP-A-62-123167 (corresponding to U.S. Pat. No. 4,709,024) and JP-A-63-54358 (corresponding to U.S. Pat. No. 4,717,769) disclose the use of high silica metallosilicate catalysts for the production of xcex5-caprolactam.
Processes for producing xcex5-caprolactam, which are not based on the Beckmann rearrangement, are also known. For example, JP-A-2-215767 discloses a process comprising the step of cycling methyl6-aminocaproate to obtain xcex5-caprolactam, U.S. Pat. No. 5,495,016 discloses a process comprising the step of reacting 6-aminocapronitrile with water to obtain xcex5-caprolactam, and JP-A-9-3041 discloses a process comprising the step of reacting methyl 6-hydroxycaproate with ammonia in the presence of hydrogen and steam to obtain xcex5-caprolactam.
Also, crude xcex5-caprolactam is obtained in a process of depolymerizing, at a high temperature, oligomers and polymers containing Nylon-6 which are recovered in the production of Nylon-6.
However, xcex5-caprolactam obtained by the above-described processes contains various impurities. As is well known, xcex5-caprolactam is used as a raw material for the preparation of polyamide, and the xcex5-caprolactam utilized to prepare polyamide for producing synthetic fibers or films is required to have high purity. Thus, the xcex5-caprolactam prepared by the above processes, that is the crude xcex5-caprolactam containing impurities, is first purified by various purification methods such as crystallization, extraction, distillation and hydrogenation to obtain a high purity xcex5-caprolactam, which is then utilized.
Among the purification methods, crystallization is known to be a method in which quite a few kinds of impurities can be removed all at once. However, a generally known crystallization method such as a crystallization with cooling has problems. For example, when the crystallization with cooling is conducted continuously, effective removal of heat of the crystallization is needed. Therefore, the difference in temperature between an xcex5-caprolactam solution and a medium for cooling needs to be large such that a tank for crystallization and/or crystallizer needs to have a huge cooling surface area. In addition, since xcex5-caprolactam deposits (so-called scales) are easily produced at the cooling surface of the tank or the crystallizer, a specific tank or crystallizer is needed such as a crystallization vessel with a scraper (which is used for the removal of scales) or a multistage crystallizer in which the difference in temperature between an xcex5-caprolactam solution and a medium for cooling is small to reduce the scale generation on an inner wall of the crystallizer. Such specific tank and crystallizer are expensive and make a plant cost high.
JP-A-1-261,363 discloses another crystallization method of reducing the pressure in a tank in which the crystallization is conducted, to utilize latent heat of evaporation for removing the heat of crystallization therein. This method also needs specific equipment such as a vacuum pump and a vacuum crystallizer and, therefore, again makes plant cost high.
The present inventors have conducted extensive research with the object of providing a process for producing an xcex5-caprolactam having a high quality, the process industrially removing impurities continuously from crude xcex5-caprolactam in an efficient and economical way. As a result, it has been found that such a production process is provided by purifying crude xcex5-caprolactam in a crystallization method in which molten, crude xcex5-caprolactam and a cooled solvent are poured into a vessel. The present invention has been accomplished on the bases of the findings.
The present invention provides a process for producing xcex5-caprolactam, comprising the steps of:
(i) pouring molten, crude xcex5-caprolactam and a solvent into a vessel, the solvent comprising an aliphatic hydrocarbon and having a temperature lower than the temperature of the crude xcex5-caprolactam, and mixing the xcex5-caprolactam and solvent to obtain a first slurry containing crystallized xcex5-caprolactam, and
(ii) subjecting the first slurry to a solid-liquid separation to obtain xcex5-caprolactam and a first liquid phase.